1. Field of the Invention
The present invention relates to a switch box.
2. Description of the Related Art
More devices in more fields have been electronized recently, and a plurality of secondary batteries are used as a power supply to satisfy the demand for a sufficient amount of electricity.
For example, provided to some automobiles is an in-vehicle power supply system using a lithium-ion battery as a sub-battery for supplying power to an in-vehicle equipment, such as an audio device, in addition to a lead storage battery serving as a main battery for driving the starter motor for starting the engine.
In addition to the demand for electricity, some other reasons for the use of the in-vehicle power supply system include the demands for improving the lifetime of devices and avoiding cost increase.
Lead storage batteries as the main battery, which are less expensive than the lithium-ion batteries as a sub-battery, are less durable against frequent charging/discharging. In particular, the lead storage battery deteriorates quickly due to frequent charging/discharging in, for example, vehicles with idling stop function, which stops the engine when the vehicles are stopping, or vehicles generating and storing electric power with an alternator using the regenerative energy of the vehicles.
Therefore, the less expensive lead storage battery is used in combination with the lithium-ion battery, which is more durable against frequent charging/discharging. The lithium-ion battery is preferentially used in operations requiring frequent charging/discharging, such as an operation of supplying power for a load during idling stop and regenerative charging, so that the lead storage battery deteriorates less. The lead storage battery is then used to store power for backup power supply for a long time period, for example, so as to avoid cost increase due to use of the lithium-ion battery, which has a small capacity. A related-art example is disclosed in Japanese Patent Application Laid-open No 2009-166769. In such an in-vehicle power supply system, when one of the main battery and the sub-battery is connected with its polarities reversed after a replacement, for example, a current may flow from a battery on the higher potential side to the other battery on the lower potential side (hereinafter, such a current is referred to as a reverse connection current).
This creates a need for a switch box that allows building of an in-vehicle power supply system in which such a reverse connection current flow can be prevented reliably.